Human nails support a surprisingly varied fungal flora, including not only dermatophytes and yeasts, but many mold species both common and uncommon. These infections are known by two names, onychomycosis and tinea unguium. Onychomycosis literally means "fungus infection of the nail," where "tinea" is an ancient word for "ringworm."
Three groups of fungi are involved in onychomycosis: the dermatophytes, the molds and the yeasts. An attempt to account for the various clinical manifestations of onychomycosis by considering the requirements of these groups of fungi was made by English in "Nails and Fungi", British Journal of Dermatology, 94, 697 (1976).
Invasion of the nail apparatus by fungi produces changes of varying degree which are indistinguishable in respect to the causative fungus. A variety of other dermatoses affect nail growth and are frequently misdiagnosed as fungal infections. However, the clinical features of these nail conditions are ordinarily sufficient to distinguish them. The fungus which causes nail infections most commonly is T. rubrum. Next in importance is T. mentagrophytes, though it infects the toenails most commonly, rarely the fingernails. Other species of dermatophytes, including members of the genera Microsporum and Epidermophyton, cause nail infections uncommonly, with the possible exception of T. violaceum, which has been reported as the responsible agent in fingernail infections by observers in some Far Eastern countries. Candidiasis of the nails is another condition which occurs most frequently in individuals with immuno-deficiency diseases.
Ringworm infections of the toenails are an exceedingly common condition in races accustomed to wearing shoes. It is almost inevitable in persons who have had recurrent attacks of athlete's foot or who suffer from the chronic squamous type of foot ringworm caused by T. rubrum. It is encountered most frequently in adult males, though it is by no means uncommon in females. Ringworm of the toenails has been considered of little medical importance, since it causes no subjective distress unless the distortion is very marked, with resultant impingement of the nail margin of the soft tissues. The role of chronic infections of nails upon the general health of the individual has not been determined. Most authorities agree that it is a public health problem and it would be beneficial to eradicate the parasite. There is always the possibility that a fungal infection of the nail may act in a synergistic way in allowing pathogenic microbial agents to invade the body.
Ringworm infections of the nail organ vary greatly in their manifestations, from changes which are hardly detectable to the full-blown infection in which the nail plate is almost completely replaced by hard amorphous tissue (keratin). The infection may be entirely confined to the lateral nail groove (gutter of nail), failing to invade the substance of the nail plate at all, or doing so to a very trivial degree. This is the most minor expression of ringworm of the nail organ, and it is usually overlooked.
Further extension of the infection occurs from the lateral grooves underneath the lateral borders of the nail into the keratin produced by the nail bed. It then extends upward into the lower surface of the nail plate. The infection frequently becomes static at this point, progressing no further over a period of years, and resulting only in white or yellow discoloration and minor disorganization of the involved portion of the nail plate. If the affected zones are firm and intact, the changes do not ordinarily cause much concern to the patient.
With further progression of the disease, the fungus extends farther into the lower portion of the nail plate, and a disturbance of the nail bed results, transforming it into a thickened membrance which synthesizes keratin rapidly. This stimulation of the nail bed epithelium to produce soft keratin is the most characeteristic change in well-defined ringworm infections of the nail, since it produces an accumulation of variable amounts of subungual hyperkeratosis. This change may, however, be produced in other diseases and is not pathognomonic for ringworm infection. The fungus proliferates abundantly in this subungual mass, serving as a mycotic reservoir for extension of the infection farther into the nail plate and to other nails and to other individuals. The progression from this point on is variable. The fungus possesses an enzyme (keratinase) which splits the nail allowing further spread of infection. The nail bed may become very hyperkeratotic, lifting up and separating the nail plate. The keratinous debris may fuse with the over-riding nail plate and give the impression that the nail itself is greatly thickened. The nail may reach a thickness of 8 mm. Normally the thickness of the toenail is about 2 mm. The patient is unable to cut the nail. The end becomes irregular and jagged, and one has to resort to filing the nail.
The nail matrix also becomes involved and the lunula usually present in the large toenail disappears. The destructive changes in the nail occur after it is formed, not because of any primary disturbance in its growth. In some instances the nail plate may be very widely separated from the bed, and the curved nail becomes troublesome mechanically. Destruction of the nail plate usually occurs distally in the beginning, but may extend all the way to the proximal portion.
The treatment of onychomycosis to date is difficult. Most physicians make no attempt at treatment because hitherto there has not been any acceptable method of therapy for toenails. Systemic treatment with griseofulvin may be curative, but the drug must be taken for a period of ten to twelve month or longer, and may give rise to side effects. Even then little affect is usually noticed on badly infected toenails. Even when cure is apparent reinfection commonly occurs, particularly in plantar and interdigital tinea pedis cases.
The two volume text book Dermatology, by Moschella, et al., W. B Saunders, (1975), Chapter 13--"Superficial and Deep Mycotic Infections" at page 662 states that topical antifungal compounds are completely ineffective in reversing the changes and do not have any demonstrable effects in preventing progression of the infection. This is because the primary site of infection is under the nail plate.
The best treatment for nondermatophytic onychomycosis is avulsion which is curative in a majority of cases. This is the apparent conclusion of Dr. Zaias in "Onychomycosis", Arch. Derm., Vol. 105, pp. 263-274 (February 1972) who stated at page 273 of said article that there is no effective topical drug available at present. Even avulsion of the nail plate alone will not result in cure of fungal infections of the toenail, but must be accompanied by treatment with griseofulvin to afford complete cure.
As recent as February 1978, in an article "Imidazoles Seen Closest to Ideal Antifungal Agent" in Skin & Allergy News at page 58 it is reported that Dr. Herny E. Jones, chairman of dermatology at the Emory University School of Medicine, Atlanta, said that no topical agents will treat onychomycosis or tinea capitis and griseofulvin must be used against those infections for which topical antifungals are useless.
Avulsion of the infected toenail, i.e., surgery, is traumatic and results in loss of the nail if the matrix is removed or frequently, recurrence if the matrix is not removed. Long term treatment with griseofulvin also is not very effective for infections of the toenail. Therefore, it is still highly desirable to achieve an effective topical treatment for onychomycosis.